This invention relates to an apparatus for use with a safety rod in a nuclear reactor. More particularly, this invention relates to an apparatus for use with a safety rod in a nuclear reactor which in the event of a predetermined temperature increase will automatically release the safety rod into the core of the reactor.
A nuclear reactor typically includes a reactor vessel containing a core, said core comprising vertical elongated fuel assemblies interspersed with control and safety rods. In a liquid metal fast breeder reactor (LMFBR) the fuel assemblies are usually supported from beneath the core, while the control rods and safety rods are housed in drive shafts which are suspended from drive means disposed above the core. The drive means control the elevation of the rods with respect to the core. The fuel assemblies are disposed to provide a critical mass of nuclear fuel within the core so that a nuclear chain reaction is sustained and nuclear energy is produced. Coolant flows upwardly from the bottom of the reactor vessel, past the fuel assemblies and control rods, and out of the vessel, carrying energy produced by the chain reaction as heat to a heat exchange system.
The nuclear chain reaction is moderated by means of the control and safety rods. These rods may contain either a nuclear fuel, a neutron poison, or some combination of fuel and poison, depending on the design of the particular reactor. Regardless of composition, the rods are typically designed so that upward motion of the rods accelerates the chain reaction, and downward motion of the rods slows the chain reaction.
Under normal operating conditions, the control rods are used to adjust the reactor power level by incremental vertical movement. The safety rods are used to stop the chain reaction in the event of an abnormal occurrence which results in a substantial increase in temperature, such as a thermal excursion. Under normal operating conditions the safety rods are in a fully elevated position above the reactor core. In the event of a thermal excursion, the reactor can be rendered subcritical by lowering only a few safety rods into the reactor core.
For maximum safety, nuclear reactors are designed to anticipate and respond appropriately to breakdowns due to either equipment malfunctions or natural disasters which can result in thermal excursions. Under such emergency conditions, the primary automatic safety system of the reactor is designed to rapidly force the safety rods down into the reactor core at a rate of several feet per second. This rapid movement, known as a reactor "scram," immediately stops the nuclear chain reaction. Thus, if a thermal excursion should raise the coolant temperature a predetermined amount above its normal safe operating level, the primary automatic safety system is activated to lower the safety rods and scram the reactor. However, it would be desirable for the reactor to be safe even if the primary automatic safety system were to fail. In that case, it would be desirable to have a supplemental system which would release the safety rods downward into the core to stop the nuclear chain reaction. Preferably, such a system would operate independently of signal actuation. Ideally, such a system would operate on the basis of physical principles and would require no mechanical linkages.
Attempts have been made to design such systems. These designs are often activated by overheating of the flowing coolant or loss of coolant flow. For example, it is known to fasten safety rods to the saftey rod drive shafts by means of hydraulic couplings that lose their holding power with the decline of coolant flow. In another design, disclosed in U.S. Pat. No. 4,405,558 to Mangus et al., safety rods are fastened to safety rod drive shafts by magnets that lose their magnetic fields at certain temperatures. Still another design, set forth in U.S. Pat. No. 3,976,543 to Sowa, discloses a safety rod supported by convex bimetallic disks which invert at certain temperatures to release the safety rod. These systems have varying degrees of complexity which may affect their dependability, and also their ease of installation. A simpler, more reliable system would be desirable.